1. Field of the Invention
The present invention relates to a water base pigment ink for ink-jet recording which causes neither clog-up nor discharge failure, which imposes no load by any solidified ink on the maintenance system such as the wiping equipment, and which has good sticking performance on paper.
2. Description of the Related Art
In the ink-jet recording system, ink droplets are formed by means of the ink discharge method including, for example, the electrostatic attraction method, the method in which mechanical vibration or displacement is applied to the ink by using a piezoelectric element, and the method in which bubbles are generated by heating the ink to utilize the pressure generated thereby. All or a part of the ink droplets are adhered to a recording objective material such as paper to perform the recording.
Those used as the ink for ink-jet recording to be employed for the ink-jet recording system as described above include those in which a variety of water-soluble dyes or pigments are dissolved or dispersed in liquid media each comprising water or a combination of water and water-soluble organic solvent. It is required, for example, for the ink for ink-jet recording that neither precipitation nor aggregation appears even when the ink is not used for a long period of time, no clog-up occurs in the ink flow passage and at the tip of the head of the ink-jet printer, the printing quality is satisfactory, and the weather resistance is excellent.
The dye and the pigment may be compared with each other as follows. That is, the dye is inferior in water resistance, because the dye is completely dissolved in the ink medium composed of water or a combination of water and water-soluble organic solvent. The dye is also considerably inferior in light resistance because of its chemical structure. On the other hand, the pigment is extremely superior in weather resistance including water resistance, because the pigment is not dissolved in the ink medium but dispersed therein. In particular, the pigment ink is easily solidified after the evaporation of water. When the pigment ink is once solidified on the paper surface, the strong bonding appears between the pigment and the paper. Therefore, the pigment ink is more excellent in sticking performance than the dye ink. Further, when the pigment ink contains a resin as a dispersing agent or a binder, this function is further facilitated. Therefore, the ink for ink-jet recording is gradually changed from the dye ink to the pigment ink in recent years.
However, in the case of the pigment ink, when the water contained in the ink is evaporated to cause the solidification, then the pigment ink remains solidified, and the ink is not redissolved. Therefore, any clog-up is caused, for example, at the tip of the nozzle of the ink-jet head, and thus the discharge failure of the ink has been hitherto caused. When the ink is solidified, for example, at the cap and the wiping portion, it is difficult to perform, for example, the wiping operation. A problem arises such that the load is imposed on the maintenance system.